Förlusten gjorde supportern ledsen.

Because both nouns are in the definite singular form.

• en förlust = a loss

• förlusten = the loss

• en supporter = a supporter / a fan
• supportern = the supporter / the fan

In Swedish, the definite article is often added as a suffix instead of being a separate word.

A small extra point: supporter becomes supportern, not supporteren, because many Swedish nouns that already end in -er just add -n in the definite singular.

Because Swedish usually puts the at the end of the noun.

So instead of:

• English: the loss
• Swedish: förlusten

and instead of:

• English: the supporter
• Swedish: supportern

A separate word such as den is used when there is also an adjective:

• den stora förlusten = the big loss
• den ledsna supportern = the sad supporter

But in your sentence, there is no adjective directly attached to either noun, so the suffix form is enough.

Gjorde is the past tense of göra.

Göra can mean both do and make, depending on the context.

Here it means made, because the pattern is:

• göra någon ledsen
• make someone sad

So:

• Förlusten gjorde supportern ledsen
  = The loss made the supporter sad

The structure is:

• Förlusten = subject
• gjorde = verb
• supportern = object
• ledsen = adjective describing the object

So the pattern is:

• [something] made [someone] [adjective]

This is very similar to English:

• The loss made the supporter sad

Swedish often uses this structure with göra:

• Det gjorde mig glad = It made me happy
• Nyheten gjorde honom arg = The news made him angry

Because ledsen agrees with supportern, which is singular and common gender (an en word).

The adjective forms are:

• ledsen for common gender singular
• ledset for neuter singular
• ledsna for plural

Examples:

• Supportern var ledsen = The supporter was sad
• Barnet var ledset = The child was sad
• Supportrarna var ledsna = The supporters were sad

In your sentence, the person affected is supportern, so ledsen is the correct form.

No. Ledsen can mean both sad and sorry, depending on context.

Examples:

• Jag är ledsen can mean I am sad
• Jag är ledsen can also mean I’m sorry

In your sentence, because it is about a loss, the meaning is clearly sad, not sorry.

Not always exactly.

In Swedish, supporter very often means fan, especially in sports:

• en fotbollssupporter = a football fan

So in this sentence, supportern is most naturally understood as the fan or the supporter of a team.

In English, supporter can be broader, like a political supporter or someone who supports a cause. Swedish can use it that way too, but the sports meaning is especially common.

It can mean either, depending on context.

• förlust = loss
• in sports or competition, it can also be understood as defeat

So this sentence could be understood as:

• The loss made the supporter sad
• or more naturally in a sports context, The defeat made the fan sad

If the wider context is about a team losing a match, defeat is a very natural translation.

Yes. That is another very natural way to express a similar idea.

• Förlusten gjorde supportern ledsen
  = The loss made the supporter sad
• Supportern blev ledsen av förlusten
  = The supporter became sad because of the loss

The difference is mainly one of perspective:

• gjorde highlights the cause
• blev highlights the change of state

Both are good Swedish.

You would change gjorde to gör:

• Förlusten gör supportern ledsen
  = The loss makes the supporter sad

So:

• göra = infinitive
• gör = present
• gjorde = past

This is a useful verb to memorize because it is very common and slightly irregular.

You would use en instead of the definite noun endings:

• En förlust gjorde en supporter ledsen
  = A loss made a supporter sad

Compare:

• Förlusten gjorde supportern ledsen
  = The loss made the supporter sad
• En förlust gjorde en supporter ledsen
  = A loss made a supporter sad

This is a very useful contrast for learning Swedish articles:

• en förlust / förlusten
• en supporter / supportern